Method of releasing radio bearer in wireless communication system and receiver

ABSTRACT

A method and apparatus of releasing a radio bearer in a wireless communication system is provided. After determining to release a radio link control (RLC) entity and a packet data convergence protocol (PDCP) entity, the RLC entity is released after delivering an RLC service data unit (SDU) extracted according to a release request of the RLC entity to the PDCP entity. A first PDCP SDU obtained by processing the RLC SDU is stored in a reception buffer at the PDCP entity. The PDCP entity is released after delivering the first PDCP SDU stored in the reception buffer to a higher layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of U.S. Provisionalapplication No. 61/140,904 filed on Dec. 26, 2008, U.S. Provisionalapplication No. 61/142,257 filed on Jan. 2, 2009, and Korean PatentApplication No. 10-2009-0095995 filed on Oct. 9, 2009, all of which areincorporated by reference in their entirety herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method and apparatus for releasing a radio bearer ina wireless communication system.

2. Related Art

Wireless communication systems are widely used to provide a voiceservice or a packet service. A multiple access system supportsmulti-user communication by sharing available system resources. Examplesof the multiple access system include a code division multiple access(CDMA) system, a frequency division multiple access (FDMA) system, atime division multiple access (TDMA) system, an orthogonal frequencydivision multiple access (OFDMA) system, etc.

3-rd generation partnership project (3GPP) release 8 introduces 3GPPlong term evolution (LTE) that is an evolution of a universal mobiletelecommunication system (UMTS). The 3GPP LTE uses OFDMA in a downlink,and uses single carrier-FDMA (SC-FDMA) in an uplink. The 3GPP LTEemploys multiple input multiple output (MIMO) having 4 antennas. Inrecent years, there is an ongoing discussion on 3GPP LTE-advanced(LTE-A) that is an evolution of the 3GPP LTE.

A radio bearer is a logical path for data transmission between a userand a network. The radio bearer can be created, re-established, andreleased anytime at the request of the user or the network. In general,when the radio bearer is released, entire data stored in a buffer ofeach layer is directly discarded without performing any process on thedata.

However, if the entire data is directly discarded when the radio beareris released, it may cause ineffective use of resources. This is because,when successfully received data is discarded due to the release of theradio bearer, the data may be redundantly transmitted again thereafter.

SUMMARY OF THE INVENTION

The present invention provides a method of releasing a radio bearer toavoid a loss of data stored in a buffer, and a receiver.

In an aspect, a method of releasing a radio bearer in a wirelesscommunication system is provided. The method includes determining torelease a radio link control (RLC) entity and a packet data convergenceprotocol (PDCP) entity, releasing the RLC entity after delivering an RLCservice data unit (SDU) extracted according to a release request of theRLC entity to the PDCP entity, storing a first PDCP SDU obtained byprocessing the RLC SDU in a reception buffer at the PDCP entity, andreleasing the PDCP entity after delivering the first PDCP SDU stored inthe reception buffer to a higher layer.

The method may further include delivering a second PDCP SDU previouslystored in the reception buffer to the higher layer.

The plurality of PDCP SDUs stored in the reception buffer may bedelivered to the higher layer in an ascending order of a sequence number(SN) of each PDCP SDU.

The plurality of PDCP SDUs stored in the reception buffer may be allPDCP SDUs stored in the reception buffer.

The PDCP entity may obtain the first PDCP SDU by performing decipheringand header decompression on the RLC SDU.

The RLC SDU may be obtained by reassembling at least one RLC protocoldata unit (PDU) received from a transmitter.

The number of RLC SDUs delivered to the PDCP entity may be a pluralnumber, and the plurality of RLC SDUs may be delivered to the PDCPentity in an ascending order of an SN of each RLC SDU.

In another aspect, a receiver includes a radio frequency (RF) unit, anda processor, operatively coupled to the RF unit, for releasing a radiobearer, and configured to determine to release a radio link control(RLC) entity and a packet data convergence protocol (PDCP) entity,release the RLC entity after delivering an RLC service data unit (SDU)extracted according to a release request of the RLC entity to the PDCPentity, store a first PDCP SDU obtained by processing the RLC SDU in areception buffer at the PDCP entity, and release the PDCP entity afterdelivering the first PDCP SDU stored in the reception buffer to a higherlayer.

A radio resource can be effectively utilized by avoiding redundant datatransmission. In addition, a loss of data blocks discontinuouslyreceived can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structure of a wireless communication system.

FIG. 2 is a block diagram showing functional split between an evolveduniversal terrestrial radio access network (E-UTRAN) and an evolvedpacket core (EPC).

FIG. 3 is a diagram showing a radio protocol architecture for a userplane.

FIG. 4 is a diagram showing a radio protocol architecture for a controlplane.

FIG. 5 is a flow diagram showing hybrid automatic repeat request (HARQ)and automatic repeat request (ARQ).

FIG. 6 is a flowchart showing an example of releasing a radio bearer.

FIG. 7 is a conceptual diagram showing a process of releasing a radiobearer.

FIG. 8 is a block diagram showing wireless communication system toimplement an embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 is a block diagram showing a wireless communication system. Thismay be a network structure of a 3rd generation partnership project(3GPP) long term evolution (LTE)/LTE-advanced (LTE-A). An E-UTRAN(Evolved-UMTS Terrestrial Radio Access Network) includes at least onebase station (BS) 20 providing a user plane and a control plane towardsa user equipment (UE) 10. The UE can be fixed or mobile and can bereferred to as another terminology, such as a MS (Mobile Station), a UT(User Terminal), a SS (Subscriber Station), MT (mobile terminal), awireless device, or the like. The BS 20 may be a fixed station thatcommunicates with the UE 10 and can be referred to as anotherterminology, such as an e-NB (evolved-NodeB), a BTS (Base TransceiverSystem), an access point, or the like. There are one or more cellswithin the coverage of the BS 20. Interfaces for transmitting usertraffic or control traffic can be used between BSs 20. The BSs 20 areinterconnected with each other by means of an X2 interface. The BSs 20are also connected by means of the S1 interface to the EPC (EvolvedPacket Core), more specifically to the MME (Mobility Management Entity)by means of the S1-MME and to the Serving Gateway (S-GW) by means of theS1-U. The S1 interface supports a many-to-many relation between MME/S-GW30 and the BS 20.

Hereinafter, downlink means communication from the BS 20 to the UE 10,and uplink means communication from the UE 10 to the BS 20. In downlink,a transmitter may be a part of the BS 20 and a receiver may be a part ofthe UE 10. In uplink, a transmitter may be a part of the UE 20 and areceiver may be a part of the BS 20.

FIG. 2 is a block diagram showing functional split between the E-UTRANand the EPC. Slashed boxes depict radio protocol layers and white boxesdepict the functional entities of the control plane. ABS hosts thefollowing functions. (1) Functions for Radio Resource Management such asRadio Bearer Control, Radio Admission Control, Connection MobilityControl, Dynamic allocation of resources to UEs in both uplink anddownlink (scheduling), (2) IP (Internet Protocol) header compression andencryption of user data stream, (3) Routing of User Plane data towardsS-GW, (4) Scheduling and transmission of paging messages, (5) Schedulingand transmission of broadcast information, and (6) Measurement andmeasurement reporting configuration for mobility and scheduling. The MMEhosts the following functions. (1) NAS (Non-Access Stratum) signaling,(2) NAS signaling security, (3) Idle mode UE Reachability, (4) TrackingArea list management, (5) Roaming and (6) Authentication. The S-GW hoststhe following functions. (1) Mobility anchoring and (2) lawfulinterception. The PDN gateway (P-GW) hosts the following functions. (1)UE IP (internet protocol) allocation and (2) packet filtering.

FIG. 3 is a block diagram showing radio protocol architecture for a userplane. FIG. 4 is a block diagram showing radio protocol architecture fora control plane. The data plane is a protocol stack for user datatransmission and the control plane is a protocol stack for controlsignal transmission.

Referring to FIGS. 3 and 4, a physical (PHY) layer provides informationtransfer services to upper layers on a physical channel. The PHY layeris coupled with a MAC (Medium Access Control) layer, i.e., an upperlayer of the PHY layer, through transport channels. Data is transferredbetween the MAC layer and the PHY layer through the transport channel.The transport channels are classified by how and with whatcharacteristics data are transferred over the radio interface. Betweendifferent physical layers, i.e., the physical layer of a transmitter andthe physical layer of a receiver, data are transferred through thephysical channel.

There are several physical control channels used in the physical layer.A physical downlink control channel (PDCCH) may inform the UE about theresource allocation of paging channel (PCH) and downlink shared channel(DL-SCH), and hybrid automatic repeat request (HARQ) information relatedto DL-SCH. The PDCCH may carry the uplink scheduling grant which informsthe UE about resource allocation of uplink transmission. A physicalcontrol format indicator channel (PCFICH) informs the UE about thenumber of OFDM symbols used for the PDCCHs and is transmitted in everysubframe. A physical Hybrid ARQ Indicator Channel (PHICH) carries HARQACK/NAK signals in response to uplink transmissions. A physical uplinkcontrol channel (PUCCH) carries uplink control information such as HARQAC/NAK in response to downlink transmission, scheduling request andchannel quality indicator (CQI). A physical uplink shared channel(PUSCH) carries uplink shared channel (UL-SCH).

The functions of the MAC layer include mapping between logical channelsand transport channels, and multiplexing/demultiplexing of MAC SDUs(Service Data Units) belonging to one or different logical channelsinto/from transport blocks (TBs) delivered to/from the PHY layer ontransport channels. The MAC layer provides services to a RLC (Radio LinkControl) layer through logical channels. Logical channels may beclassified into two groups: control channels for the transfer of controlplane information and traffic channels for the transfer of user planeinformation.

The functions of the RLC layer include concatenation, segmentation andreassembly of RLC SDUs. In order to guarantee various quality ofservices (QoSs) required by radio bearers (RBs), the RLC layer providesthree operating modes: TM (Transparent Mode), UM (Unacknowledged Mode)and AM (Acknowledged Mode). The AM RLC provides error correction throughautomatic repeat request (ARQ).

The functions of a PDCP (Packet Data Convergence Protocol) layer for theuser plane include transfer of user data, headercompression/decompression and ciphering/deciphering. The functions ofthe PDCP layer for the control plane include transfer of control planedata, and ciphering and integrity protection.

A RRC (Radio Resource Control) layer is defined only in the controlplane. The RRC layer serves to control the logical channels, thetransport channels and the physical channels in association withconfiguration, reconfiguration and release of radio bearers (RBs). A RBmeans a logical path provided by a first layer (i.e. PHY layer) andsecond layers (i.e. MAC layer, RLC layer and PDCP layer) for datatransmission between a UE and a network. Configuring the RB includesdefining radio protocol layers and characteristics of channels toprovide a service and defining specific parameters and operationschemes. The RB may be classified into a signaling RB (SRB) and a dataRB (DRB). The SRB is used as the path to transfer RRC messages in thecontrol plane and the DRB is used as the path to transfer user data inthe user plane.

A NAS (Non-Access Stratum) layer belonging to the upper layer of the RRClayer serves to perform session management and mobility management.

FIG. 5 is a flow diagram showing hybrid automatic repeat request (HARM)and automatic repeat request (ARQ). An RLC entity 110 of a transmitter100 transmits an RLC protocol data unit (PDU) with a sequence number(SN) of 0 to a MAC entity 120 (step S110). The MAC entity 120 of thetransmitter 100 transmits a MAC PDU1(s) corresponding to the RLC PDUwith SN=0 to a MAC entity 220 of a receiver 200 (step S112). Uponsuccessfully receiving the MAC PDU, the MAC entity 220 transmits the RLCPDU with SN=0 to an RLC entity 210 (step S114).

The RLC entity 110 of the transmitter 100 transmits an RLC PDU with SN=1to the MAC entity 120 (step S120). The MAC entity 120 of the transmitter100 transmits a MAC PDU2(s) corresponding to the RLC PDU with SN=1 tothe MAC entity 220 of the receiver 200 (step S122). It is assumed that aradio channel deteriorates and thus HARQ is performed on the MAC PDU2.

The RLC entity 110 of the transmitter 100 transmits an RLC PDU with SN=2to the MAC entity 120 (step S130). The MAC entity 120 of the transmitter100 transmits a MAC PDU3(s) corresponding to the RLC PDU with SN=2 tothe MAC entity 220 of the receiver 200 (step S132). Upon successfullyreceiving the MAC PDU3, the MAC entity 220 transmits the RLC PDU withSN=2 to the RLC entity 210 (step S134).

An HARQ process performed on the MAC PDU2(s) corresponding to the RLCPDU with SN=1 is complete (step S135), and the RLC entity 210 obtainsthe RLC PDU with SN=1 (step S136). According to the HARQ process, SNs ofRLC PDUs received by the RLC entity 210 may not be continuous. This isreferred to as HARQ reordering.

The RLC entity 110 of the transmitter 100 transmits an RLC PDU with SN=3to the MAC entity 120 (step S140). In this case, a polling bit is set ina header of the RLC PDU, and a status report is requested. The MACentity 120 of the transmitter 100 transmits a MAC PDU4(s) correspondingto the RLC PDU with SN=3 to the MAC entity 220 of the receiver 200 (stepS142). Upon successfully receiving the MAC PDU4, the MAC entity 220transmits the RLC PDU with SN=3 to the RLC entity 210 (step S144). Whenthe status report is requested, the RLC entity 210 constructs a statusPDU and transmits the status PDU to the RLC entity 110 of thetransmitter 100 (step S150).

A PDU is a data block transmitted by a certain layer to a lower layer.An SDU is a data block received by a certain layer from a higher layer.For example, an RLC PDU is a data block transmitted by an RLC to a MAC,and an RLC SDU is a data block received by the RLC from a PDCP.Conversion from the PDU to the SDU or conversion from the SDU to the PDUmay differ depending on functions of the layers.

An RLC entity supporting ARQ is referred to as an acknowledged mode (AM)RLC entity. An RLC entity not supporting ARQ is referred to as anunacknowledged mode (UM) RLC entity.

The UM RLC entity constructs each PDU having a header including asequence number (SN) to allow the UM RLC entity of the receiver to beable to know which PDU is lost in transmission. In the UM RLC, a userplane handles broadcast/multicast data transmission or real-time packetdata transmission, such as voice (e.g., voice over Internet protocol(VoIP)) or streaming in a packet service domain, and a control planehandles transmission of an RRC message which does not require areception acknowledgement among RRC messages transmitted to a specificUE or a specific UE group in a cell.

After receiving a PDU, the UM RLC entity of the receiver first examinesan SN of the PDU. If the received PDU is an in-sequence PDU with respectto a previously received PDU, an SDU obtained by processing the PDU isdelivered to a higher layer (e.g., PDCP). Otherwise, if the received PDUis an out-of-sequence PDU, the PDU is stored in a buffer. The PDU waitsin the buffer until the in-sequence PDU is received.

The RLC entity receives a PDU out of sequence for the following tworeasons. A first reason is that the PDU is lost in transmission, and asecond reason is that HARQ reordering occurs in a lower layer. Whenout-of-sequence reception occurs due to the PDU loss as described in thefirst reason, the UM RLC entity of the receiver preferably transmits aPDU received out of sequence to the higher layer as soon as possible.When out-of-sequence reception occurs due to the HARQ reordering asdescribed in the second reason, an in-sequence PDU is received after aspecific time elapses. Thus, the UM RLC of the receiver preferably waitsuntil the specific time elapses, and then receives PDUs in sequence andprocesses the received PDUs.

A timer is defined by the UM RLC entity of the receiver to deal with theaforementioned both problematic cases of receiving the PDU out ofsequence. When a certain PDU is received out of sequence, the PDU isstored in a buffer and the timer is started at the same time. If anin-sequence PDU is not received until the timer expires, it isdetermined that out-of-sequence reception occurs due to a PDU loss. Thatis, when the certain PDU is received out of sequence, the PDU waits inthe buffer until the timer expires or until a previous in-sequence PDUis received.

Similarly to the UM RLC entity, the AM RLC entity also constructs a PDUwith a header including an SN. However, the AM RLC entity differs fromthe UL RLC entity in that the AM RLC entity transmits an acknowledgmentto the transmitter in response to the received PDU. The acknowledgmentis used to request the transmitter to retransmit a PDU that fails to bereceived by the receiver. The AM RLC is used for the purpose ofguaranteeing error-free data transmission by performing retransmission.In the AM RLC, a user plane handles non-real-time packet datatransmission such as transmission control protocol/Internet protocol(TCP/IP) in a packet service domain, and a control plane handlestransmission of an RRC message that requires a reception acknowledgmentamong RRC messages transmitted to a specific UE in a cell.

Upon receiving a PDU, the AM RLC entity also first examines an SN of thePDU, which is the same as in the UM RLC entity. If the received PDU isan in-sequence PDU with respect to a previously received PDU, an SDUobtained by processing the PDU is delivered to a higher layer.Otherwise, if the received PDU is an out-of-sequence PDU, the PDU isstored in a buffer. The PDU waits in the buffer until the in-sequencePDU is received.

The AM RLC receives a PDU out of sequence for the same reason as in theUM RLC. However, since the AM RLC supports retransmission, the PDUreceived out of sequence is not delivered when the timer expires.Instead, a status report is transmitted to the AM RLC of the transmitterto request retransmission of an unsuccessfully received PDU. Therefore,in the AM RLC entity, if a certain PDU is received out of sequence, thereceive PDU waits in a buffer until a previous in-sequence PDU isreceived.

A PDCP layer receives a PDCP PDU delivered from an RLC layer that is alower layer of the PDCP layer. Then, the PDCP layer performs decipheringand header decompression on the PDCP PDU, and delivers a decompressedPDCP SDU to a higher layer. In general, a case where the PDCP SDU waitsin a buffer of the PDCP layer does not occur. This is because an UM RLCor an AM RLC which is a lower layer of the PDCP layer always deliversthe PDCP PDU in an ascending order of an SN, and thus the PDCP deliversthe received PDCP PDU to the higher layer by processing the PDCP PDU inorder.

A radio bearer (RB) is a path for data transmission, and can be createdand released anytime at the request of a UE or a network. However, asdescribed above, PDUs or SDUs may be stored in the RLC buffer and/or thePDCP buffer due to out-of-sequence reception such as HARQ reordering. Inthis case, if data stored in the RLC buffer and/or the PDCP buffer isdirectly discarded when the RB is released, it may cause ineffectiveresource utilization.

Therefore, in a method described below, data stored in a buffer can beprocessed when an RB is released so that a data loss is minimized evenwhen the RB is released.

FIG. 6 is a flowchart showing an example of releasing a radio bearer(RB). This method can be performed by a processor of a receiver.

In step S610, the receiver determines to release the RB. An RRCtransmits a release request of the RB to an RLC and a PDCP to requestrelease of the RLC and the PDCP. The RLC may be a UM RLC or an AM RLC.

In step S620, the RLC of the receiver obtains an RLC SDU by reassemblingthe RLC PDU stored in an RLC reception buffer, and delivers thereassembled RLC SDU to the PDCP. The RLC of the receiver removes aheader from an RLC PDU (e.g., a UMD PDU, an AMD PDU, and/or an AMD PDUsegment) which is successfully received and which is stored in the RLCreception buffer, and reassembles the RLC PDU, thereby obtaining acomplete RLC SDU. If the obtained RLC SDU is incomplete due to a lostPDU, the incomplete RLC SDU is discarded. The reassembled RLC SDU isdelivered to the PDCP. The delivered RLC SDU corresponds to the PDCPPDU. The reassembled RLC SDU may be delivered to the PDCP in anascending order of an SN.

In step S630, the RLC SDU is delivered to the PDCP, and thereafter theRLC is released.

In step S640, the PDCP of the receiver obtains a PDCP SDU by processinga PDCP PDU stored in the PDCP reception buffer and/or a PDCP PDUdelivered from the RLC, and thereafter delivers the obtained PDCP SDU toa higher layer. The PDCP obtains a complete PDCP SDU by performingdeciphering and header decompression on the PDCP PDU stored in the PDCPreception buffer and the PDCP PDU delivered from the RLC, and thenstores the obtained PDCP SDU in the PDCP reception buffer. All completedPDCP SDUs stored in the PDCP reception buffer are delivered to thehigher layer of the PDCP. The higher layer of the PDCP may be anapplication layer or the like which can receive and process the PDCPSDU. The PDCP SDU may be delivered to the higher layer in an ascendingorder of an SN.

In step S650, the PDCP SDU is delivered to the higher layer, andthereafter the PDCP is released.

The RB may be released in an abnormal situation. For example, this is acase where a channel state between a UE and a network temporarilydeteriorates or a radio resource is temporarily insufficient. Byallowing previously received data to be delivered to the higher layerwhen the RB is released, the previous data can be maintained and a dataloss can be prevented when the RB is reestablished.

Although release of the RLC and the PDCP is described herein, thetechnical features of the present invention may also apply when releaseis performed for at least two layers. For example, in the abovedescription, under the assumption that a first layer is the RLC layerand a second layer is the PDCP layer, a data block is delivered whenthese two layers are released. The technical features of the presentinvention can also apply when release is performed for layersconstituting another radio interface protocol.

FIG. 7 is a conceptual diagram showing a process of releasing an RB. AnRRC transmits a release request to an RLC and a PDCP to release the RB.In the RLC, an RLC PDU 702 with SN=6, an RLC PDU 703 with SN=7, and anRLC PDU 705 with SN=9 are stored in an RLC reception buffer, and an RLCPDU 701 with SN=5 and an RLC PDU 704 with SN=8 wait to be received. Inresponse to the RLC release request, the RLC extracts a first RLC SDU711 from the RLC PDU 702 with SN=6 and the RLC PDU 703 with SN=7, andextracts a second RLC SDU 712 from the RLC PDU 705 with SN=9. Theextracted first RLC SDU 711 and the extracted second RLC SDU 712 aredelivered to the PDCP, and thereafter the RLC is released.

The first RLC SDU 711 and second RLC SDU 712 delivered to the PDCPbecome a first PDCP PDU and a second PDCP PDU, respectively. The PDCPobtains a first PDCP SDU 721 and a third PDCP SDU 723 by performingdeciphering and header decompression on each of the first RLC SDU 711and second RLC SDU 712 delivered from the RLC, and stores the obtainedSDUs in a PDCP reception buffer. Further, the PDCP obtains a second PDCPSDU 722 from a PDCP PDU previously stored in the PDCP reception buffer,and stores the obtained SDU in the PDCP reception buffer. The first PDCPSDU 721, the second PDCP SDU 722, and the third PDCP SDU 723 aredelivered to a higher layer of the PDCP, and thereafter the PDCP isreleased.

A radio resource can be effectively utilized by avoiding redundant datatransmission. In addition, a loss of data blocks discontinuouslyreceived can be prevented.

FIG. 8 is a block diagram showing wireless communication system toimplement an embodiment of the present invention. A transmitter 50 and areceiver 60 may be a part of a user equipment, a base station or a relaystation. A transmitter 50 may include a processor 51, a memory 52 and aradio frequency (RF) unit 53. The processor 51 may be configured toimplement proposed functions, procedures and/or methods described inthis description. Layers of the radio interface protocol may beimplemented in the processor 51. The memory 52 is operatively coupledwith the processor 51 and stores a variety of information to operate theprocessor 51. The RF unit 53 is operatively coupled with the processor11, and transmits and/or receives a radio signal. A receiver 60 mayinclude a processor 61, a memory 62 and a RF unit 63. The processor 61may be configured to implement proposed functions, procedures and/ormethods described in this description. The memory 62 is operativelycoupled with the processor 61 and stores a variety of information tooperate the processor 61. The RF unit 63 is operatively coupled with theprocessor 61, and transmits and/or receives a radio signal.

The processors 51, 61 may include application-specific integratedcircuit (ASIC), other chipset, logic circuit and/or data processingdevice. The memories 52, 62 may include read-only memory (ROM), randomaccess memory (RAM), flash memory, memory card, storage medium and/orother storage device. The RF units 53, 63 may include baseband circuitryto process radio frequency signals. When the embodiments are implementedin software, the techniques described herein can be implemented withmodules (e.g., procedures, functions, and so on) that perform thefunctions described herein. The modules can be stored in memories 52, 62and executed by processors 51, 61. The memories 52, 62 can beimplemented within the processors 51, 61 or external to the processors51, 61 in which case those can be communicatively coupled to theprocessors 51, 61 via various means as is known in the art.

In view of the exemplary systems described herein, methodologies thatmay be implemented in accordance with the disclosed subject matter havebeen described with reference to several flow diagrams. While forpurposed of simplicity, the methodologies are shown and described as aseries of steps or blocks, it is to be understood and appreciated thatthe claimed subject matter is not limited by the order of the steps orblocks, as some steps may occur in different orders or concurrently withother steps from what is depicted and described herein. Moreover, oneskilled in the art would understand that the steps illustrated in theflow diagram are not exclusive and other steps may be included or one ormore of the steps in the example flow diagram may be deleted withoutaffecting the scope and spirit of the present disclosure.

What has been described above includes examples of the various aspects.It is, of course, not possible to describe every conceivable combinationof components or methodologies for purposes of describing the variousaspects, but one of ordinary skill in the art may recognize that manyfurther combinations and permutations are possible. Accordingly, thesubject specification is intended to embrace all such alternations,modifications and variations that fall within the spirit and scope ofthe appended claims.

1. A method of releasing a radio bearer in a wireless communicationsystem, the method comprising: determining to release a radio linkcontrol (RLC) entity and a packet data convergence protocol (PDCP)entity; releasing the RLC entity after delivering an RLC service dataunit (SDU) extracted according to a release request of the RLC entity tothe PDCP entity; storing a first PDCP SDU obtained by processing the RLCSDU in a reception buffer at the PDCP entity; and releasing the PDCPentity after delivering the first PDCP SDU stored in the receptionbuffer to a higher layer.
 2. The method of claim 1, further comprisingdelivering a second PDCP SDU previously stored in the reception bufferto the higher layer.
 3. The method of claim 2, wherein the plurality ofPDCP SDUs stored in the reception buffer are delivered to the higherlayer in an ascending order of a sequence number (SN) of each PDCP SDU.4. The method of claim 2, wherein the plurality of PDCP SDUs stored inthe reception buffer are all PDCP SDUs stored in the reception buffer.5. The method of claim 1, wherein the PDCP entity obtains the first PDCPSDU by performing deciphering and header decompression on the RLC SDU.6. The method of claim 1, wherein the RLC SDU is obtained byreassembling at least one RLC protocol data unit (PDU) received from atransmitter.
 7. The method of claim 1, wherein the number of RLC SDUsdelivered to the PDCP entity is a plural number, and the plurality ofRLC SDUs are delivered to the PDCP entity in an ascending order of an SNof each RLC SDU.
 8. A receiver comprising: a radio frequency (RF) unit;and a processor, operatively coupled to the RF unit, for releasing aradio bearer, and configured to: determine to release a radio linkcontrol (RLC) entity and a packet data convergence protocol (PDCP)entity; release the RLC entity after delivering an RLC service data unit(SDU) extracted according to a release request of the RLC entity to thePDCP entity; store a first PDCP SDU obtained by processing the RLC SDUin a reception buffer at the PDCP entity; and release the PDCP entityafter delivering the first PDCP SDU stored in the reception buffer to ahigher layer.
 9. The receiver of claim 8, wherein the processor isfurther configured to deliver a second PDCP SDU previously stored in thereception buffer to the higher layer.
 10. The receiver of claim 9,wherein the plurality of PDCP SDUs stored in the reception buffer aredelivered to the higher layer in an ascending order of a sequence number(SN) of each PDCP SDU.
 11. The receiver of claim 10, wherein theplurality of PDCP SDUs stored in the reception buffer are all PDCP SDUsstored in the reception buffer.
 12. The receiver of claim 10, whereinthe processor is configured to obtain the first PDCP SDU by performingdeciphering and header decompression on the RLC SDU at the PDCP entity.13. The receiver of claim 8, wherein the RLC SDU is obtained byreassembling at least one RLC protocol data unit (PDU) received from atransmitter.
 14. The receiver of claim 8, wherein the number of RLC SDUsdelivered to the PDCP entity is a plural number, and the plurality ofRLC SDUs are delivered to the PDCP entity in an ascending order of an SNof each RLC SDU.